Am 02.07.2018 um 10:23 schrieb Daniel Vetter:
On Fri, May 04, 2018 at 03:47:59PM +0200, Daniel Vetter wrote:
On Fri, May 04, 2018 at 03:17:08PM +0200, Christian König wrote:
Am 04.05.2018 um 11:25 schrieb Daniel Vetter:
On Fri, May 4, 2018 at 11:16 AM, Chris Wilson <chris@xxxxxxxxxxxxxxxxxx> wrote:
Quoting Daniel Vetter (2018-05-04 09:57:59)
On Fri, May 04, 2018 at 09:31:33AM +0100, Chris Wilson wrote:
Quoting Daniel Vetter (2018-05-04 09:23:01)
On Fri, May 04, 2018 at 10:17:22AM +0200, Daniel Vetter wrote:
On Fri, May 04, 2018 at 09:09:10AM +0100, Chris Wilson wrote:
Quoting Daniel Vetter (2018-05-03 15:25:52)
Almost everyone uses dma_fence_default_wait.
v2: Also remove the BUG_ON(!ops->wait) (Chris).
I just don't get the rationale for implicit over explicit.
Closer approximation of dwim semantics. There's been tons of patch series
all over drm and related places to get there, once we have a big pile of
implementations and know what the dwim semantics should be. Individually
they're all not much, in aggregate they substantially simplify simple
drivers.
I also think clearer separation between optional optimization hooks and
mandatory core parts is useful in itself.
A new spelling of midlayer ;) I don't see the contradiction with a
driver saying use the default and simplicity. (I know which one the
compiler thinks is simpler ;)
If the compiler overhead is real then I guess it would makes to be
explicit. I don't expect that to be a problem though for a blocking
function.
I disagree on this being a midlayer - you can still overwrite everything
you please to. What it does help is people doing less copypasting (and
assorted bugs), at least in the grand scheme of things. And we do have a
_lot_ more random small drivers than just a few years ago. Reducing the
amount of explicit typing just to get default bahaviour has been an
ongoing theme for a few years now, and your objection here is about the
first that this is not a good idea. So I'm somewhat confused.
I'm just saying I don't see any rationale for this patch.
"Almost everyone uses dma_fence_default_wait."
Why change?
Making it look simpler on the surface, so that you don't have to think
about things straight away? I understand the appeal, but I do worry
about it just being an illusion. (Cutting and pasting a line saying
.wait = default_wait, doesn't feel that onerous, as you likely cut and
paste the ops anyway, and at the very least you are reminded about some
of the interactions. You could even have default initializers and/or
magic macros to hide the cut and paste; maybe a simple_dma_fence [now
that's a midlayer!] but I haven't looked.)
In really monolithic vtables like drm_driver we do use default
function macros, so you type 1 line, get them all. But dma_fence_ops
is pretty small, and most drivers only implement a few callbacks. Also
note that e.g. the ->release callback already works like that, so this
pattern is there already. I simply extended it to ->wait and
->enable_signaling. Also note that I leave the EXPORT_SYMBOL in place,
you can still wrap dma_fence_default_wait if you wish to do so.
But I just realized that I didn't clean out the optional release
hooks, I guess I should do that too (for the few cases it's not yet
done) and respin.
I kind of agree with Chris here, but also see the practical problem to copy
the default function in all the implementations.
We had the same problem in TTM and I also don't really like the result to
always have that "if (some_callback) default(); else some_callback();".
Might be that the run time overhead is negligible, but it doesn't feels
right from the coding style perspective.
Hm, maybe I've seen too much bad code, but modeset helpers is choke full
of exactly that pattern. It's imo also a trade-off. If you have a fairly
specialized library like ttm that's used by relatively few things, doing
everything explicitly is probably better. It's also where kms started out
from.
But if you have a huge pile of fairly simple drivers, imo the balance
starts to tip the other way, and a bit of additional logic in the shared
code to make all the implementations a notch simpler is good. If we
wouldn't have acquired quite a pile of dma_fence implementations I
wouldn't have bothered with all this.
So ack/nack on this (i.e. do you retract your original r-b or not)? It's
kinda holding up all the cleanup patches below ...
Feel free to add my Acked-by for now, but I still have a kind of a gut
feeling that we might want to revisit this decision at some time.
Christian.
I went ahead and applied the first three patches of this series meanwhile.
-Daniel
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